With the continuing development of computer electronics technology, new electronic packages such as the latest CPUs can perform more and more functions. Heat generated by these modem electronic packages has increased commensurately. The heat has become an impediment for continuous development of the CPUs. Cooling fans used for helping heat dissipation of these electronic packages are required to provide increasing airflow, which is usually measured by a unit of CFM (Cubic Feet Per Minute). To increase the CFM value of airflow passing these electronic packages, increasing rotational speed of the cooling fans is a useful way.
Another fact not to harmonize with the development of the CPUs is that, in spite of the high speed of the latest CPUs, other peripheral devices that incorporate with the CPUs, such as hard disk drives or optical disk drives, run at a relatively slower speed. Unfortunately, these slowly operated peripheral devices, not the fast-operated CPUs, determine the overall performance of computer systems. Therefore, to enhance the overall performance of the computer system, it is desired to improve the operation speed of the disk drives. This improvement requires a higher rotational speed of motors of the disk drives.
In any one of these cooling fans or peripheral devices, a brushless direct current motor is usually included to drive a blade set or a disk. A typical brushless direct current motor comprises a stator and rotor pivotably installed about the stator. The blade set of the cooling fan or the disk of the disk drive is carried by the rotor to run. In order to increase the rotational speed of the cooling fan or the disk drive, the motor must output a larger torque to drive the blade set or the disk to run faster.
On the other hand, in addition to being used in computer field, brushless direct current motors are increasingly utilized in home appliances such as refrigerators or air conditioners, office instruments such as copy machines, printers or the like. Employment of more and more motors consumes considerably large electrical energy. Nowadays, saving energy is an important issue for designs of electronic products. Therefore, it is desired to increase the efficiency of the motors of transferring electrical energy to rotational mechanical energy.
For the foregoing reasons, there is a need for a brushless direct current motor which can provide an improved torque and has an improved efficiency.